AuntPetunia..unfortunately dogs and cats under the law are considered property and the destruction of property only becomes a felony if the monetary value is significant. The laws need to change though because dogs and cats are family members.

You are wrong. His beating the cat did not directly cause his death. Felony murder rules apply when the commission of the felony directly causes the death. As LR 1955 noted, the wife did not have to kill him; she could have just moved out with the cat. Even if he had been beating a child, she could only intervene at the time of the beating in defense of the person. You just can't kill people because they are mean.